Blanket statements like this aren't very useful in either direction, for or against app killers. They have their uses if used properly and if the user is armed with the knowledge to use one. That said, they can also cause problems if the user doesn't know what's going on and assumes that Android operates the same way as the user's former device.

For lack of knowledge, because I don't own an android device right now, does the OS swap to flash when its low on memory? If so, is there a way to tweak swappiness in the kernel? If it is swapping I could see it running slower. It could also be a CPU contention issue and killing off apps would help a bit, depending on what the other apps are doing.